A belt press, in which a continuous long object to be pressed is put on a press belt, and the object to be pressed is pressed between one pressing member positioned inside a periphery of the press belt and the other pressing member positioned outside the periphery of the press belt, has been used in various kinds of industries. Here, the pressing means includes a press roll and a pressure shoe. For example, the belt press includes a shoe press as a dehydrating press in the papermaking industry.
The shoe press will be briefly described, taking the papermaking industry for example. That is, the shoe press is a pressing (dehydrating) operation in which a surface pressure is applied to an object to be pressed (wet paper web) on an outer periphery of a press belt between a press roll positioned outside a periphery of the press belt serving as external pressing means and a pressure shoe positioned inside the periphery of the press belt serving as internal pressing means through the press belt. While a linear pressure is applied to the object to be pressed in the roll press using two rolls, a area pressure can be applied to the object to be pressed using the pressure shoe having a predetermined width in a travel direction in the shoe press. Therefore, when a dehydrating press is performed by the shoe press, since a nip width can be large, hydrating efficiency can be improved.
In order to make the shoe press compact, a shoe press roll in which a pressure shoe serving as internal pressing means is covered with a flexible cylindrical press belt (press jacket) to be assembled to have a rolled shape has been widely used as disclosed in Japanese Unexamined Patent Publication No. 61-179359, for example.
Characteristics required for the press belt includes strength, abrasion resistance, flexibility and impermeability to water, oil, gas and the like in general. As a material comprising the above characteristics, polyurethane provided by a reaction between urethane prepolymer and a curing agent has been used for the press belt in general.
In a papermaking technique, it is known that many drain grooves extending along a belt travel direction are formed in an external surface of the press belt in order to drain water squeezed from the pressed wet paper web.
FIG. 10 is a sectional view showing a conventional press belt having typical drain grooves. An illustrated press belt 80 comprises many drain grooves 81 extending along a belt travel direction, and many lands positioned between the adjacent drain grooves and extending along the belt travel direction. Each of the drain groove 81 and the land 82 has a rectangular section in a belt width direction in general.
FIG. 11 shows a state in which a wet paper web 84 to be pressed and a felt 83 are sandwiched between the press belt 80 and a press roll 85. This state is a state before pressed. An upper surface of the land 82 is flat and this flat upper surface is in surface contact with the felt 83.
As the press is performed from the state shown in FIG. 11, an upper part of the land 82 is pressed downward and swelled sideways as shown in FIG. 12B. Thus, an opening of the drain groove is reduced in size and a dehydrating performance (draining performance) is lowered.
FIG. 12A shows a pressure distribution on the wet paper web, corresponding to FIG. 12B. According to the pressure distribution on the wet paper web, although a pressure is not applied to a part above the drain groove 81, a high pressure is applied to a part on the land 82. Since the land 82 has the flat upper surface, the same high pressure is applied to a part having a certain degree of width and the pressure is abruptly reduced at both ends of the width, that is, a boundary between the drain groove 81 and the land 82.
Such large pressure difference causes a paper web component to be changed. More specifically, an orientation difference of fiber, a yield difference of a filler, a volume difference and the like are generated. Since such change in paper web component is quite noticeable at the part on the boundary between the drain groove 81 and the land 82, a groove mark extending along the belt travel direction appears at this boundary. The groove mark lowers a paper quality.
In addition, since the flat upper surface of the land 82 comes in surface contact with the wet paper web 84 through the felt 83 at high pressure, water is captured in the flat upper surface and cannot be drained to the drain groove, so that the dehydrating performance could deteriorate. In addition, as shown in FIG. 12B, since the drain groove is large on the bottom but small in the middle part, the water in the drain groove 81 is not likely to be discharged. Thus, the press belt 80 containing water comes in contact with the wet paper web again because the water is not drained. When such phenomenon is generated, the wet paper web is not dehydrated and the paper is further moistened.
According to a press belt disclosed in Japanese Utility Model Publication No. 1-36960, in order to prevent an opening of a drain groove from being closed because a press belt is deformed at the time of pressing, a lateral width of an opening of the groove is formed wider than that of the bottom of the groove. According to a press belt disclosed in Japanese National Publication of International Application No. 10-510594, in order to reduce closure of a drain groove at the time of pressing, both side walls of the groove gradually upwardly diverge. In addition, a press belt disclosed in Japanese Unexamined Patent Publication No. 11-335992, in order to maintain a water retention volume as large as possible, a slope to increase an opening width is formed in an opening region of a side wall of a drain groove.
Even when the press belt has drain grooves to improve the draining performance, since the land between the grooves has the flat upper surface, the paper component is inevitably changed due to the large pressure difference at the boundary between the land and the groove. In addition, the problem in which water is captured in the flat upper surface cannot be solved.